Many foolhardy collapse (5)
I believe the answer is:
crash
'collapse' is the definition.
(I know that collapse can be written as crash)
'many foolhardy' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'foolhardy' becomes 'rash' (synonyms).
'c'+'rash'='CRASH'
(Other definitions for crash that I've seen before include "Economic collapse" , "failure" , "Dramatic fall" , "(Of shares) fall suddenly by a great amount" , "Impact" .)